As it is well known in the art, a fuel injection system of an internal combustion engines typically includes a plurality of fuel injectors, each of which is arranged to inject fuel into the combustion chamber of an associated engine cylinder. Each fuel injector is supplied with high-pressure fuel from a suitable source, such as a common rail, which is charged with fuel at high pressure by a high-pressure fuel pump.
A fuel injector generally comprises a nozzle body, which houses an elongate valve needle. The nozzle body has a spray tip with one or more spray orifices, and, in use, the injector is mounted so that its spray tip protrudes into the associated combustion chamber, whereby fuel can be injected into the combustion chamber at high pressure in the form of an atomised spray. Fuel supplied from a high-pressure fuel channel accumulates in the nozzle body, surrounding the needle, ready for delivery to the spray orifices when required for injection.
The longitudinal displacement of the needle is controlled by means of a solenoid-actuated control valve arrangement, which controls the pressuring or discharging of a control chamber located above the valve needle. For example, the end of the needle opposite the tip is received within the control chamber and is thus subject to the fuel pressure therein, causing a pressure force in the closing direction. In addition, a spring mounted e.g. in the closing chamber conventionally biases the needle in the closing direction.
The control chamber is supplied with fuel from the high-pressure fuel channel through an inlet restrictor, which defines an inlet flow rate. Fuel can exit the control chamber through an outlet channel leading to the control valve and further downstream to a low-pressure drain, when the control valve is open. An outlet restrictor is provided at the entry of the outlet channel to control the flow rate of fuel exiting the control chamber. In order to open the injector, the solenoid actuator of the control valve arrangement is energized to move its valve member and hence open the flow path towards the low-pressure drain, causing a pressure drop in the control chamber. Injection occurs when the pressure acting on the needle spray region exceeds the force exerted on the needle by the fuel in the control chamber and the spring force, causing the needle to lift.
Such fuel injector is, e.g., disclosed in EP 2 647 826.
Fuel injectors of the above-described type have shown to generally operate in a satisfactory manner. However, it has been observed that the injector performances are altered at low temperatures, e.g. when starting a cold engine under cold climate conditions, say below 0° C. This appears to be mostly due to the fact that at low temperatures, the viscosity of the fuel increases substantially, causing difficulties for opening the nozzle and thus deficiency on the injector delivery rate.